Magnetoresistive random-access memory (“MRAM”) is a non-volatile memory technology that stores data through magnetic storage elements. These elements are two ferromagnetic plates or electrodes that can hold a magnetic field and are separated by a non-magnetic material, such as a nonmagnetic metal or insulator. In general, one of the plates has its magnetization pinned (i.e., a “reference layer”), meaning that this layer has a higher coercivity than the other layer and requires a larger magnetic field or spin-polarized current to change the orientation of its magnetization. The second plate is typically referred to as the free layer and its magnetization direction can be changed by a smaller magnetic field or spin-polarized current relative to the reference layer.
MRAM devices store information by changing the orientation of the magnetic moment of the free layer. In particular, based on whether the free layer is in a parallel or anti-parallel alignment relative to the reference layer, either a “1” or a “0” can be stored in each MRAM cell. Due to the spin-polarized electron tunneling effect, the electrical resistance of the cell change due to the orientation of the magnetic fields of the two layers. The cell's resistance will be different for the parallel and anti-parallel states and thus the cell's resistance can be used to distinguish between a “1” and a “0”. One important feature of MRAM devices is that they are non-volatile memory devices, since they maintain the information even when the power is off. The two plates can be sub-micron in lateral size and the magnetization can still be stable with respect to thermal fluctuations.
A newer technique, spin transfer torque or spin transfer switching, uses spin-aligned (“polarized”) electrons to change the magnetization orientation of the free layer in the magnetic tunnel junction. In general, electrons possess a spin, a quantized number of angular momentum intrinsic to the electron. An electrical current is generally unpolarized, i.e., it consists of 50% spin up and 50% spin down electrons. Passing a current through a magnetic layer polarizes electrons with the spin orientation corresponding to the magnetization direction of the magnetic layer (i.e., polarizer), thus produces a spin-polarized current. If a spin-polarized current is passed to the magnetic region of a free layer in the magnetic tunnel junction device, the electrons will transfer a portion of their spin-angular momentum to the magnetization layer to produce a torque on the magnetization of the free layer. Thus, torque can switch the magnetization of the free layer, which, in effect, writes either a “1” or a “0” based on whether the free layer is in the parallel or anti-parallel states relative to the reference layer.
FIG. 1 illustrates a magnetic tunnel junction (“MTJ”) stack 100 for a conventional MRAM device. As shown, stack 100 includes one or more seed layers 110 provided at the bottom of stack 100 to initiate a desired crystalline growth in the above-deposited layers. A pinning layer 112 is disposed on top of seed layers 110 and a synthetic antiferromagnetic layer (“SAF layer”) 120 is disposed on top of the pinning layer 112. Furthermore, MTJ 130 is deposited on top of SAF layer 120. MTJ 130 includes the reference layer 132, a barrier layer (i.e., the insulator) 134, and the free layer 136. It should be understood that reference layer 132 is actually part of SAF layer 120, but forms one of the ferromagnetic plates of MTJ 130 when the barrier layer 134 and free layer 136 are formed on reference layer 132. The first magnetic layer in the synthetic antiferromagnetic structure 120 is exchange coupled to the pinning layer 112, which causes, through antiferromagnetic coupling, the magnetization of the reference layer 132 to be fixed. Furthermore, a nonmagnetic spacer 140 is disposed on top of MTJ 130 and a perpendicular polarizer 150 is disposed on top of the nonmagnetic spacer 140. Perpendicular polarizer 150 is provided to polarize a current of electrons (“spin-aligned electrons”) applied to MTJ structure 100. Further, one or more capping layers 160 can be provided on top of perpendicular polarizer 150 to protect the layers below on MTJ stack 100. Finally, a hard mask 170 is deposited over capping layers 160 and is provided to pattern the underlying layers of the MTJ structure 100, using a reactive ion etch (RIE) process.
MRAM products having MTJ structures, such as stack 100 illustrated in FIG. 1, are already being used in large data storage devices. In order to instantaneously initiate the magnetization reversal of the storage layer (i.e., the free layer), such MTJ structures utilize perpendicular polarizers whose magnetization is orthogonal to the storage layer. One critical limitation with such designs is that the final magnetic vector state of the storage layer cannot be controlled.
One proposed solution to control the final magnetic vector state is to have a first current polarity to start the magnetization reversal process and a second current polarity to stop the magnetization precession of the free layer at a defined magnetization state. However, implementation of this technique/design is not yet possible due to technological limitations of pulse control, i.e., in the range of 100 picoseconds. In addition, the non-magnetic conductor layers in the conventional MTJ designs are inadequate to obtain high tunneling magnetoresistance value (“TMR”) and to achieve the switching characteristics that are required from such devices. Another proposed solution to control the final magnetic state of the storage layer is to have the spin torque from the reference layer be greater than the spin torque from the polarizer. However, this design is only theoretical in nature and has not been successfully manufactured to date.
In addition, effective MTJ structures require large switching currents that limit their commercial applicability. There are at least two critical parameters that control the required size of the switching current: effective magnetization Meff and the damping constant for the free layer structure. Some existing designs have attempted to lower the required switching current by reducing the thickness of the free layer structure. While such a design facilitates a perpendicular component of the magnetization that effectively lowers the Meff, the measurable reduction of Meff only occurs when the free layer is very thin (e.g., 1 nanometer). However, such a thin free layer has severe consequences including: (1) a significant reduction of tunneling magnetoresistance value (“TMR”); (2) a lower thermal stability; and (3) an increased damping constant for the free layer.
FIG. 2 illustrates a table comparing the TMR value versus thickness of a CoFeB free layer for a conventional MTJ structure with a copper (Cu) nonmagnetic spacer 140. As shown, the MTR value for a conventional MTJ structure with a 2.3 nm CoFeB free layer is approximately 80%. As is readily apparent, when the thickness of the free layer decreases to decrease the switching current, the TMR value rapidly decreases, for example, to a TMR value of 9% for a CoFeB free layer thickness of 1.5 nm. As further shown, even a CoFeB free layer having a thickness of 1.8 nm provides a device with a TMR value of approximately 38%.
These TMR values are completely inadequate for MRAM applications. In practice, a TMR value of approximately 120% or greater is required to meet the MRAM requirements and specifications. Prior art OST-MTJ structures simply cannot achieve this high TMR and also have inferior switching characteristics due to: (i) the spacer layers used (such as Cu) between the free layer and the polarizer (i.e., a nonmagnetic spacer 140 of FIG. 1); and (ii) poor free layer magnetic properties.